Electrode catalysts in which a catalyst metal containing platinum is supported on a carbon carrier have been mainly used for solid polymer fuel cells. However, carbon carriers that have been used as catalyst carriers are easily oxidized during high-potential operation such as start-stop operation of fuel cells. Therefore, for example, the supported catalyst metals aggregate and separate, which mainly causes the degradation of the performance of electrode catalysts.
In order to reduce the load of auxiliaries of fuel cell systems, a fuel cell is desirably operated while a low-humidification gas is supplied. However, if the fuel cell is operated under the low-humidification conditions, the proton conductivity of ionomers in an electrolyte membrane and a catalyst layer is degraded because of a dry-up phenomenon, which decreases the output of the fuel cell.
Patent literature 1 discloses that, in order to improve the durability of carbon carriers during high-potential operation, the degree of graphitization of carbon carriers is increased through heat treatment to suppress the oxidation of the carbon carriers during the operation of fuel cells. However, only the heat treatment of carbon carriers is hard to achieve high oxidation resistance of carbon carriers during high-potential operation. In particular, it is difficult to prevent the degradation of the performance of fuel cells during operation under low-humidification conditions. Furthermore, if a carbon carrier having a high degree of graphitization is used, the corrosion resistance improves, but the specific surface area of the carbon carrier decreases, which degrades the power generation performance.
Patent literature 2 discloses an electrode catalyst obtained by mixing a carbon carrier that supports a catalyst metal (platinum) and an acidic oxide in order to improve the durability while maintaining high activity of a catalyst. However, the carbon carrier is highly hydrophobic and the acidic oxide is highly hydrophilic, and thus it is very difficult to physically mix such catalyst carriers having different physical properties in a uniform manner. The nonuniform physical mixing causes a variation in the catalytic performance and also causes performance degradation by, for example, unevenness and cracking during coating of a catalyst layer.
Patent literature 3 discloses that a catalyst material containing hydrophilic particles of zeolite, titanium dioxide, or the like is used for an anode in order to maintain the cell performance to some degree during operation under low-humidification conditions. However, such a catalyst material does not exhibit electric conductivity, and thus the internal resistance of the catalyst layer is expected to increase.
Patent literature 4 discloses a carbon material for carriers of fuel cells, the carbon material being obtained by mixing a moisture-retentive carbon material and carbon black. In this technique, the occlusion and release of water vapor by an activated carbon material can be expected to some degree. However, the water retention property is not sufficiently imparted to ionomers in an electrolyte membrane and a catalyst layer during the operation of a fuel cell under low-humidification conditions, and thus high cell output cannot be achieved. Furthermore, the carbon material used in Patent literature 4 is a material into which micropores are introduced to impart the water retention property to the carbon material. Therefore, the carbon material is very easily oxidized during high-potential operation of fuel cells, and the durability may be degraded.
Patent literature 5 discloses an electrode catalyst obtained by calcining zirconium and a carbon material precursor. However, a high cell output cannot be achieved using a catalyst that does not contain a noble metal such as Pt.